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I think it's atan(2*mu), which here is 45deg. Post a solution in a bit.

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Nope.

Let me just give the numerical answer so you'll know when you've gotten it.

The slope is 7/4.

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Oops I think I made one other silly mistake. I get this if I modify one line in my solution:

Instead of:
d²x/dt² = a_x = r/2*alpha

If I use:
d²x/dt² = a_x = r*alpha

I get a slope of 7/4 or angle of 60.3 deg.